1. Field of the Invention
The present invention relates to a screen process printing apparatus for fabricating printed circuit boards, hybrid circuits, etc. In particular, the present invention relates to a screen process printing apparatus with a screen which is reinforced by a plurality of strips.
2. Description of the Related Art
A plate for use in screen process printing is prepared by forming figures or patterns consisting of openings and non-open portions on a screen which is stretched on a screen frame. The figures or patterns are formed mainly by a photomechanical process. Printing liquid (such as printing paste, printing ink, etc.) is put on the plate. A squeegee is pressed against and slid over the surface of the screen. This causes the ink to be squeezed out from the openings such that the figures or patterns are transferred or printed onto a surface of a material which is placed under the screen.
Referring to FIG. 2, a screen frame 1 includes a rectangular frame body 2 of wood or metal, and a screen 3 which is attached to the frame body 2. The four sides of the screen 3 are fixed to the frame body 2 with a prescribed tension by adhesives or the like.
Referring to FIG. 1, when printing is to be carried out, a material 5 to be printed is placed on a printing table 4 (by vacuum contact or other method). The frame body 2 is fixed such that a gap d exists between the material 5 and the screen 3. The screen frame 1 is set in the main body of a printing apparatus. The screen 3 is stretched horizontally as shown by the chain line. Ink 6 is applied to the screen 3. The screen 3 is pressed and bent by the squeegee 7 so that the screen 3 contacts the surface of the material 5. The screen 3 is stretched as shown by the solid lines. While the squeegee 7 moves in the direction shown by the arrow P, the ink 6 is transferred onto the material 5 through the openings in the screen 3. As the squeegee 7 advances, the position of contact between the screen 3 and the material 5 moves. The screen 3 adheres to the material 5 at the contact position due to viscosity of the ink. However, when the squeegee 7 moves further, the tension of the screen 3 causes the screen 3 to separate from the material 5. This phenomenon is called snapping off.
Japanese Patent Applications 63-108312 and 63-110856 relate to improving the snapping-off characteristics of a screen frame. The applications also relate to means for prolonging the life of a screen. The applications disclose a screen frame in which a screen is stretched between a fixed frame and a movable frame which is parallel to the fixed frame. Only two sides of the screen are fixed to the frames. Since the techniques disclosed in the Japanese applications are added to the present invention, specific examples thereof are described in the following with reference to FIGS. 3 to 6.
A screen frame 10 shown in FIG. 3 includes a fixed frame body 12. The body 12 includes a mounting frame portion 14 and two side frame portions 16 which are provided at two ends of the mounting frame portion 14. The portions 16 extend perpendicularly with respect to the portion 14. The frame 10 further includes a pair of guide rods 18 which are fixed to the respective side frame portions 16 and a movable frame body 20 which is parallel to the mounting frame portion 14. The movable frame body 20 has through-holes at its two ends. The guide rods 18 are slidably inserted through the through-holes perpendicular to the frame body 20. The frame 10 further includes a pair of compression coil springs 24. The springs 24 are interposed between the ends of the side frame portions 16 and the movable frame body 20.
The guide rods 18 extend from the ends of the side frame portions 16. The rods 18 are parallel to each other and are perpendicular to the mounting frame portion 14. Bearings (not shown) are provided in the movable frame body 20 to enable the movable frame body 20 to move smoothly in slidable contact with the guide rods 18 Stoppers 26 are fixed to the top ends of the rods 18 on the side of the movable frame body 20. The stoppers 26 limit the extent to which the movable frame body 20 can move.
The springs 24 bias the movable frame body 20 away from the fixed frame body 12 so that tension is applied to the screen 22.
The screen 22 is formed of artificial fiber (such as nylon or polyester) or a metallic mesh of stainless steel or the like. The screen can be formed of wirelike portions constituting a mesh in the case of a screen which is formed by a plating method. The wirelike portions can be regarded as wires although such a mesh is not formed by knitting respective wires.
The body 12 is fixed to a base body (by fixation screws or the like) before the frame 10 is mounted in the main body of a printing apparatus. The movable frame body 20 is supported by the rods 18. In some cases, an extension coil spring 28 having one end fixed to a fixed portion 30 of the main body of the printing apparatus may be provided. The other end of the spring 28 is fixed to the movable frame body 20. The spring 28 is used to adjust the tension of the screen when, for example, the force of the springs 24 is insufficient or it is necessary to change the tension of the screen according to requirements of printing quality.
The movable frame body 20 is biased away from the fixed frame body 12 by the springs 24 and the spring 28. The screen 22 is caused to have a predetermined tension. Referring to FIG. 3, the solid lines show the state of the movable frame body 20 and the screen 22 before printing.
Referring to FIG. 4, a screen frame 50 according to another example comprises a fixed frame body 52 and a movable frame body 54. The bodies 52 and 54 are spaced-apart and parallel. A screen 56 is stretched between the bodies 52 and 54. The frame 50 further includes a pair of guide bars 58 and an extension coil spring 60 which is attached to he movable frame body 54.
The guide bars 58 are parallel to each other. One end of each of the guide bars 58 is inserted into corresponding ends of the fixed frame body 52. Through-holes are provided in the movable frame body 54 and the other ends of the guide bar 58 penetrate through the through-holes. The movable frame body 54 is slidably engaged with the guide bars 58 through bearings 62 so that the body 54 is parallel to the body 52.
The fixed frame body 52 is fixed by fixation screws 66 to a frame holding portion 64 provided in the main body of the printing apparatus. One top end of each of the bars 58 is fixed to the frame body 52. At the same time, the other ends of the bars are fixed by screws 70 to a guide bar holding portion 68 which is also provided in the main body of the printing apparatus. One end of the extension coil spring 60 is fixed to the movable frame body 54 and the other end thereof is fixed to the fixed portion 72. The spring 60 pulls the movable frame body 54 away from the body 52 and applies tension to the screen 56.
Referring to FIGS. 5 and 6, a screen frame 80 of another example includes a rectangular fixed frame body 82. The body 82 includes mounting frame portion 84, two side frame portions 86 and a coupling frame portion 88. The frame 80 further includes a movable frame body 90 which is located inside of the body 82 and which is spaced apart from and parallel to the mounting frame portion 84. Both ends of the movable frame body 90 are rotatably supported by the side frame portions 86. A torsion coil spring 94 tends to rotate the movable frame body 90 in a counterclockwise direction to apply tension to the screen 92.
The movable frame body 90 is supported by the two side frame portions 86 through a rotating shaft 96. The shaft 96 is fixed to the movable frame body 90 through bearings (not shown) provided in the side frame portions 86. The spring 94 is wound on an outer portion of one end of the rotating shaft 96. One end of the torsion coil spring 94 engages with an engagement pin 98 which is provided in one side frame portion 86. The movable frame portion 90 is a cylindrical body. The portion 90 has a flat surface portion 100 which is formed by cutting along the longitudinal direction of the shaft, as shown in FIG. 6. The screen 92 is fixed to the movable frame body 90 with one side of the screen 92 being attached to the flat surface portion 100 by adhesives or the like.
Bias stretching is normally used to stretch the screen between the frame bodies of FIGS. 3-5. That is, the screen is stretched in such a manner that threads or wires constituting the screen are oblique with respect to the frame bodies. Since the frame bodies are perpendicular to the printing direction, i.e., the running direction of the squeegee, the threads or wires are oblique with respect to the running direction of the squeegee. In a bias stretched screen, some of the threads of the screen 22 end at the edge 102 of the screen 22 and do not extend between the two frame bodies (FIG. 7). As a result, distribution of tension over the whole area of the screen is liable to be uneven.
To avoid unevenly distributing tension, the screen can be reinforced as illustrated in FIG. 8. Referring to FIG. 8, strips 120 for reinforcing a screen frame 110 are formed on both sides of a screen 116 which is stretched between a fixed frame body 112 and a movable frame body 114 along the printing direction of a squeegee 122. A printing plate portion 118 is between the strips 120 The side edges of the i screen 116 are reinforced by the strips 120. Balancing the distribution of tension throughout the screen permits stable printing to be carried out.
Various reinforcing methods may be considered. In a first method, a photosensitive material is applied to the whole surface of a screen in a manufacturing process of a screen process printing plate and a figure or a pattern is formed in a central portion of the screen by a photomechanical process. During this manufacturing process, two side edges of the screen are reinforced by the photosensitive material remaining on the surface of the screen. The second method is to attach metallic belts to a screen. The third method is to form metallic strips on the surface of a screen by plating.
However, when the screen frame 110 is used during screen process printing, disadvantages may occur. Referring to FIG. 9, the width W0 of the squeegee 122 is smaller than the width W1 of the screen 116 (corresponding to a dimension between the reinforcing strips 120) (W0&lt;W1). The center of the squeegee 122 (in the widthwise direction) substantially coincides with the center of the screen 116 (in the widthwise direction) at the time of printing.
The reinforcing strips 120 and other portions (including the printing plate portion 118) tend to stretch differently in the squeegee running direction even when the same tension is applied thereto. An excess of force is applied to the screen 116 at the junctions between the reinforcing strips 20 and the other portions of the screen 116. The screen 16 is bent at these junctions. This causes disadvantages such as breakage of the screen 116. Such disadvantages are especially liable to occur if the strips 120 are formed by plating metallic strips on the screen 116, or if the reinforcing strips 120 are formed simultaneously with preparation of the screen by plating.